1. Field of the Invention
This invention relates to a misfire detection system for an internal combustion engine, and more particularly to such a system in which misfire detection is inactivated under specified operating conditions.
2. Description of the Prior Art
The cause for misfiring in an internal combustion engine may be related to either the fuel supply system or the ignition system of the engine. The assignee previously proposed a system for detecting misfire caused by the fuel supply system (Japanese Laid-Open Patent Publication No. 5(1993)-65866) and a system designed to improve misfire detection accuracy by prohibiting the making of misfire detections when engine combustion is apt to become unstable, as when the fuel injection system controls the air/fuel to be lean, or when the fuel supply is being cut (Japanese Laid-Open Patent Publication No. 5(1993)-164033) .
In a vehicle equipped with an automatic transmission, the engine output is sometimes deliberately reduced, such as by retarding the ignition timing, by making the air/fuel ratio lean or by cutting off the fuel supply, in order to reduce the shock the passengers feel during gear ratio shifting. Similar engine output reduction control is also conducted for mitigating swaying oscillation of the propeller shaft or the like caused by output transmission lag. Since combustion becomes unstable during control of this kind, the probability of erroneous misfire detection is high. This degrades the reliability of the misfire detection system.